Hacker News Books

Watterson writes about this in one of the bigger compilation books (one like The Days Are Just Packed): infringers are too numerous for him to track down, and he was loathe to hire on assistants just to hire IP. I couldn't find that source, but I did find him talking at length about why there's almost no officially licensed merchandise:

http://iheartpicturebooks.blogspot.ca/2010/04/bill-watterson...

I think if he had gone down the official licensing road, someone else would have taken care of infringers for him. As it is, he wanted to do what he loved and not worry about that aspect of the business.